This invention relates in general to electrostatography and, more specifically, to a novel photoconductive device and process for using the device.
In the art of xerography, a xerographic plate containing a photoconductive insulating layer is imaged by first uniformly electrostatically charging its surface. The plate is then exposed to a pattern of activating electromagnetic radiation which selectively dissipates the charge in the illuminated areas of the photoconductive insulator while leaving behind an electrostatic charge pattern in the nonilluminated areas. This resulting electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic marking particles on the surface of the photoconductive insulating layer.
A photoconductive layer for use in xerography may be a homogeneous layer of a single material such as vitreous selenium or it may be a composite layer containing a photoconductor and another material. One type of composite photoconductive layer used in xerography is illustrated in U.S. Pat. No. 4,265,990 which describes a photosensitive member having at least two electrically operative layers. One layer comprises a photoconductive layer which is capable of photogenerating holes and injecting the photogenerated holes into a contiguous charge transport layer. Generally, where the two electrically operative layers are supported on a conductive layer with the photoconductive layer sandwiched between the contiguous charge transport layer and a supporting conductive layer, the outer surface of the charge transport layer is normally charged with a uniform charge of a negative polarity and the supporting electrode is utilized as an anode. Obviously, the supporting electrode may also function as an anode when the charge transport layer is sandwiched between the anode and a photoconductive layer which is capable of photogenerating electrons and injecting the photogenerated electrons into the charge transport layer. The charge transport layer in this embodiment, of course, must be capable of supporting the injection of photogenerated electrons from the photoconductive layer and transporting the electrons through the charge transport layer.
Various combinations of materials for charge generating layers (CGL) and charge transport layers (CTL) have been investigated. For example, the photosensitive member described in U.S. Pat. No. 4,265,990 utilizes a charge generating layer in contiguous contact with a charge transport layer comprising a polycarbonate resin and one or more of certain diamine compounds. Various generating layers comprising photoconductive layers exhibiting the capability of photogeneration of holes and injection of the holes into a charge transport layer have also been investigated. Typical photoconductive materials utilized in the generating layer include amorphous selenium, trigonal selenium, and selenium alloys such as selenium-tellurium, selenium-tellurium-arsenic, selenium-arsenic, and mixtures thereof. The charge generation layer may comprise a homogeneous photoconductive material or particulate photoconductive material dispersed in a binder. Other examples of homogeneous and binder charge generation layer are disclosed, for example, in U.S. Pat. No. 4,265,990. Additional examples of binder materials such as poly(hydroxyether) resins are taught in U.S. Pat. No. 4,439,507. The disclosures of the aforesaid U.S. Pat. Nos. 4,265,990 and 4,439,507 are incorporated herein in their entirety. Photosensitive members having at least two electrically operative layers as disclosed above provide excellent images when charged with a uniform negative electrostatic charge, exposed to a light image and thereafter developed with finely divided electroscopic marking particles. However, when the supporting conductive substrate comprises a charge injecting metal or non-metal, difficulties have been encountered with these photosensitive members due to discharge in the dark. More specifically, these photosensitive members do not retain sufficient charge during the charging and subsequent imaging exposure and development steps. Most metallic ground planes have a natural oxide layer which inhibits charge injection. Typical metals of this type are aluminum, zirconium, titanium and the like. Some exceptions are metals that do not oxidize such as the noble metals, e.g., gold, platinum and the like that promote charge injection. Ground planes containing other materials such as copper iodide or carbon black also inject charge into charge generation layers so the photoreceptor does not effectively hold charge during the charging, image exposure and/or development steps. Copper iodide ground planes as disclosed in, for example, U.S. Pat. No. 4,082,551 encounter degradation problems during cycling.
When ground planes contain conductive particles dispersed in a resin binder, difficulties can be encountered with migration of the resin binder and/or conductive particles into subsequently applied layers that contain solvents which at least partially dissolve or swell the resin binder in the conductive layer. Such migration of the resin binder or conductive particles can adversely affect the integrity of the ground plane and the electrical properties of the ground plane and/or the subsequently applied layers. More specifically, polymers in the binders utilized for ground planes can migrate into the charge generating layer and cause charge trapping. When charge trapping occurs during cycling, internal fields build up and background prints out in the final printed copies. Further, conductive particles can move up to subsequently applied layers and prevent the photoreceptor from receiving a full electrostatic charge in the areas where the conductive material migrated. For example, migration of conductive particles such as carbon black into subsequently applied layers causes lower charge acceptance and perhaps V.sub.R cycle-up. The regions of lower charge acceptance appear as white spots in the final printed copy. Solvent attack can also cause discontinuities in the ground plane resulting in non-uniform charging which ultimately causes the formation of distorted images in the final toner image. Cross-linking of the resin binder in the ground plane reduces solubility. However, existing methods of cross-linking polymers such as hydroxylic polymers, although chemically efficient in the cross-linking process itself, leave much to be desired in applications for photoreceptors because of catalytic or process residues which can permanently reside in the photoreceptor. Such residues, even at the parts per million level, are very often deleterious to one or more of the sensitive electrical properties required for superior photoreceptor performance. Moreover cross-linking mechanisms often require an additional heating step which can result in the evolution of a volatile matter and residue formation. Also, cross linking capability may require an externally added low molecular weight cross-linking agent, which may not be totally consumed and may in part migrate to other layers in the photoreceptor. Also the addition of cross-linking materials or catalysts can adversely affect electrical and physical properties of the ground plane.
Charge blocking layers are frequently used on metalized or other kinds of ground planes to inhibit charge injection. Some charge blocking layers require an additional adhesive layer between the charge generation layer and the conductive ground plane. When attempts are made to use resins as a blocking layer, the photoreceptors usually exhibit increased residual charge with cycling. A catalyst is usually used to form polymers or to cross-link polymers employed in blocking layer applications. Catalytic residue present is undesirable and cause electrical problems such as charge trapping which increases residual charge with cycling ultimately leading to background deposit build up. The polymers utilized in blocking layers can mix with materials in subsequently applied layers due to sensitivity to solvents used in the subsequently applied layers. This mixing of polymers from the blocking layers with subsequently applied layers can cause the blocking layer to lose its effectiveness as a blocking function and the photoreceptor becomes unusable for forming images. Failure to effectively hold charge during the image exposure and development steps or increased residual charge formation with cycling cannot normally be tolerated in precision copiers, duplicators, and printers.
Copolymers of methyl vinyl ether and maleic anhydride such as the Gantrez AN resins from GAF Corporation have been utilized in blocking layers. Unfortunately, these copolymers of methyl vinyl ether and maleic anhydride are sensitive to water and rapidly hydrolyze to form acidic products which are corrosive and attack metal ground planes of photoreceptors during electrical cycling. Loss of the ground plane due to corrosion during electrical cycling eventually prevents an electrophotographic imaging member from discharging. This is manifested by an increase in background toner deposits in the final image during electrical cycling. In addition, the mechanical properties of copolymers of methyl vinyl ether and maleic anhydride are affected at high humidity and cause flexible electrophotographic imaging members to delaminate. Under low humidity conditions, blocking layers containing copolymers of methyl vinyl ether and maleic anhydride or maleic anhydride tend to cause electrical surface potential cycle down. Cycle down affects the final copy by causing loss of electrical contrast between exposed and unexposed areas. In addition, copolymers of methyl vinyl ether and maleic anhydride are sensitive to certain solvents utilized in subsequently applied layers and redissolve and lose integrity as a blocking layer. Hydrolysis of copolymers of methyl vinyl ether and maleic anhydride transforms the anhydride to the acid. The acid formed during storage will attack the metallic conductive layer and result in photoreceptors that will no longer discharge. Moreover, during cycling, corrosion of thin metal ground planes is accelerated and this will also result in photoreceptors that will no longer discharge. Also, when the acid is formed, coating with the material is generally restricted to coating with water and low molecular weight alcohols.
When some resins are employed in blocking layers, difficulties can be encountered with migration of the resin into subsequently applied layers that contain solvents which at least partially dissolve or swell the resin in the blocking layer. Such migration of the resin can adversely affect the integrity and the electrical properties of the blocking layer and/or the subsequently applied layers. Cross-linking of the resin in the blocking layer reduces solubility but an additional heating step to effect cross-linking may be necessary. Also the addition of cross-linking materials or catalysts may be required and these additives can adversely affect electrical and physical properties of the blocking layer.
Poly(vinylalcohol) (PVOH) has been evaluated for use as a blocking layer. However, aqueous solutions of this material are very viscous and difficult to apply as a coating. For example, very dilute but still viscous poly(vinylalcohol) aqueous solutions require numerous spray coating passes to build up blocking layer dry thickness to the desired level. Moreover, the solvents that may be employed for poly(vinylalcohol) are not conducive to the formation of high quality coatings. In addition, the adhesion of poly(vinylalcohol) to many conductive layer polymers is poor.